- Metabolism

	We are now going to take another look at metabolism, by examining a catabolic process that provides us with Energy.
	The overall metabolic process of sugar breakdown into Energy is called Respiration. We will focus on only one part of the process called Glycolysis. Glycolysis means sugar- splitting. Glycolysis is part of a universal metabolic pathway for the catabolic conversion of glucose to Energy. The process of Glycolysis is catalyzed by about ten different enzymes.

As we examine Glycolysis, remember these are biological reactions. They are catalyzed by enzymes. Kinases, dehydrogenases and isomerases are important enzymes in Glycolysis. Kinases transfer a phosphate functional group to or from ATP (a very important nucleotide triphosphate), and you are already familiar with what dehydrogenases and isomerases do. A variety of hexoses (Energy-containing sugars), including fructose and galactose can be funneled into glycolysis and broken down for energy.

The dehydrogenases: glyceraldehyde 3-phosphate dehydrogenase, lactate dehydrogenase and alcohol dehydrogenase all use NAD (a vitamin) to oxidize the substrate and obtain NADH, an important Energy-carrying molecule. These proteins contain one region that binds NAD and another domain that contains the catalytic amino acid residues.

Our bodies have several metabolic choices (e.g. anabolic or catabolic) available for the sugars we eat, dependent on the energy state of our cells. If we lay on the couch, spending all day watching the Jerry Springer show, our cells really don't need too much energy. In this instance the energy from sugar will be stored, either as Glycogen or fat, until it is needed. If we are active and our cells are in an Energy deficit, the sugars can be broken down and their Energy released in a form that our cells can use immediately

The Energy found in Glucose can be stored in several ways. Glycogen is the main storage polysaccharide of animal cells. Glycogen is a polymer of linked subunits of glucose, with branches. Glycogen is especially abundant in liver and skeletal muscle. Glycogen breakdown and synthesis are reciprocally regulated by several hormones. Insulin, for example, induces the synthesis of glycogen. Glucagon for example, binds to a receptor in the plasma membrane which activates the breakdown of glycogen inside the cell.

Another option for energy storage, especially long-term storage for large amounts of energy, is to convert the sugars to Fat. We remember that lipids and sugars have a very different chemical structure. To convert sugars to fat, our bodies have to do some simple biochemistry. The sugars are partially broken down (Glycolysis) and the two-Carbon units released from the sugars are combined into chains which form Fatty Acids.

We will briefly examine the processes of Energy conversion and its regulation, using Glycolysis as a metabolic model. Glycolysis is only one part of the catabolic process, but it will serve. We want to understand the basic of how these molecules provide Energy and how our bodies decide what to do with the energy from the foods we eat.

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